1. Field of the Invention
The present invention relates to an automatic and continuously operating standby A-C power system and more particularly to circuitry included in such a system for controlling the phase relationships between the voltage at the system's input and output terminals by the use of a single phase control loop which is fully closed when the input A-C voltage is acceptable and fully open when the input A-C voltage is not acceptable and circuitry included therein for slowly closing the loop upon the return of the input A-C voltage to acceptability after a period of nonacceptability.
2. Description of the Prior Art
For many applications it is desired to provide an automatic and continuously operating source of standby A-C power. The A-C voltage from a conventional source of power such as a power station is used to supply the load when that voltage is acceptable. Upon the occurrence of at least one of a predetermined number of conditions, such as a blackout (complete loss of A-C power), a transient (momentary voltage excursion), an interrupt (momentary loss of power) or a brownout (long-lasting reduction in A-C power), the source automatically disconnects the conventional A-C voltage from the load. Power is then supplied to the load from the standby inverter and battery contained therein. Upon restoration of the conventional or input A-C voltage the source will reconnect the line to the load and also provide current for recharging the battery. The inverter operates bidirectionally and also serves as a battery charger.
In such source it is necessary to control the phase angle between the output and conventional or input A-C voltages as a function of both the load and the charge on the battery. In this manner current may be supplied from the line voltage for providing power to the load and for charging of the battery if needed. Upon restoration of the A-C line voltage after the occurrence of one of the conditions described above, it is also desirable to slowly bring the source's output voltage into phase relationship with the A-C line before the line is reconnected to the load. This slow bringing into phase of the input and output voltages prevents undesirably large amplitude spikes from appearing at the source's output.
One such prior art source is shown and described in U.S. Pat. No. 3,348,060 (hereinafter the '060 patent) which issued on Oct. 17, 1967. A continuously adjustable phase-control circuit for use in the source described in the '060 patent is shown and described in U.S. Pat. No. 3,337,743 (hereinafter the '743 patent) which issued Aug. 22, 1967. A line control for use in the source described in the '060 patent is shown and described in U.S. Pat. No. 3,389,268 (hereinafter the '268 patent) which issued on June 18, 1968.
In the source of the '060 patent charging of the battery is controlled by circuitry which uses a lamp and a light dependent resistor (LDR). When the battery is fully charged the lamp glows dimly and the resistance of the LDR is quite high. When at least one of the predetermined conditions described above occurs and the battery is used to supply the load, the charge on the battery decreases. The lamp then begins to glow, increasing in brightness as the battery charge is reduced. The resistance of the LDR also decreases.
The phase-control system of the '743 patent also uses a lamp and an LDR as a measure and a means to control the phase difference between the source's input and output voltages. When the phase difference between the voltages is great, the lamp is relatively dark and its LDR has a relatively high resistance. As the voltages are brought substantially in phase, the lamp begins to glow brighter and the resistance of the LDR decreases.
The source described in the '060 and '743 patents includes a phase shift network consisting of an inductor and a capacitor. Both the battery charge control system and the phase control system are in the nature of closed loop sources, and the LDR's of each system are associated with the phase shift network. As both LDR's are associated with the phase shift network, they may interact under certain circumstances in a manner which if not controlled would be detrimental to the operation of the source. One such circumstance, described in the '743 patent, occurs when the battery is used to supply the load only for a short period of time as, for example, during the occurrence of a transient or an interrupt. The battery remains essentially fully charged and its lamp and LDR could dominate the lamp and LDR of the phase control circuit. The source described in the '060 and '743 patents must, therefore, include additional circuitry to prevent such detrimental interaction.
Additionally, the use of lamps as indicators of charge and phase is not desirable. Lamps have a limited lifetime and therefore require replacement. The filament of the lamps and therefore the emitted light changes with age. This aging has a degrading effect on system performance as a factory adjusted source may require a number of field adjustments during the lifetime of the lamps and upon replacement of these elements. Finally, the source described in the '060 and '743 patents is an analog system with its attendant problems of large physical size and the wide range that circuit voltages may assume.
The line control described in the '268 patent monitors the line voltage to determine when it is either excessively high or excessively low. The line control also monitors the synchronism of the standby supply to the A-C line. This monitoring is accomplished by comparing the square wave which is used to drive the inverter of the standby supply with the square wave which is used to synchronize the inverter. Upon the occurrence of any one of the conditions described above, the line control inhibits the operation of the static or line switch. Thus, the line control of the '268 patent is responsive to not only the input A-C voltage wave but other signals as well.
Any of the deficiencies associated with the prior art source may be overcome by a source which uses a single loop for controlling the phase relationship between the input and output voltages. The status of the input A-C voltage wave and the status of the battery may then be used as input signals to the loop. The status of the battery is one input which is used by the loop in controlling the phase relationship between the input and output voltages. The status of the input A-C voltage is used by the loop in determining whether the loop is operating in a closed or open condition. The operation of the source is further enhanced if the single loop uses digital techniques for controlling the phase angle between the input and output voltages.